GB2504967A

GB2504967A - Electrical supply unit with overheating protection measuring rate of change of temperature

Info

Publication number: GB2504967A
Application number: GB201214546A
Authority: GB
Inventors: Alan Radford; Ian Wedlake
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-08-15
Filing date: 2012-08-15
Publication date: 2014-02-19
Also published as: GB201214546D0

Abstract

An electrical consumer unit delivers electrical power to electrical outlet sockets has an electrical supply with live, neutral and earth lines L, N, E protected by a residual current device (RCD) 12. A thermal latch 15 responds when the rate of increase of temperature within the housing 10 exceeds a predetermined threshold level. The thermal latch 15 may also respond if the temperature exceeds a predetermined limit. The thermal latch 15 may trigger a warning device 16, which may comprise an alarm or visual warning. The warning device 16 may not reset in the event that the temperature returns to normal limits. The thermal latch 15 may produce an earth leakage current to earth 14 or neutral 13, activating the RCD 12 to isolate the unit from the power supply L, N, E. The device provides improved response to overheating over simple temperature detection.

Description

ELECTRICAL SUPPLY UNITS

TECHNICAL FIELD OF THE INVENTION

This invention relates to electrical supply units such as consumer units, wall sockets and power outlets, extension sockets, junction boxes, light switches and ceiling roses for example.

BACKGROUND

Electrical fittings such as plugs and wall sockets may sometimes be subject to overheating. As such devices age the risk tends to become greater due to an increase in electrical resistance caused by oxidation of electrical contacts, or loss of resilience in spring contacts.

Patent specification No. GB 2 446 186 A describes an electrical safety device which includes a thermal switch arranged to provide a leakage current to earth to activate a remote residual current device (RCD). The switch also triggers a thyristor latch that operates a warning device. In GB 2 474 245 A it is proposed that the electrical supply unit incorporates a two stage safety device that operates at different temperatures. A temperature-operated latch responds when the temperature within the housing exceeds a first threshold by activating a warning device. If the temperature should exceed a higher second threshold a thermal switch produces a current flow into the earth line in order to activate the RCD and thereby electrically isolate the unit from the incoming supply. The latch does not reset automatically if the temperature falls, so that the warning device continues to operate to alert people to the fact that an alarm condition has occurred.

Whilst such a two stage device is very effective under most conditions which give rise to overheating it has been found that under certain conditions a fire could start to smoulder before the temperature rise is sufficient to cut off the electrical supply.

The present invention seeks to provide a new and inventive form of safety device which effectively protects an electrical supply unit from overheating and possible fire under circumstances where a simple temperature sensor might fail to provide adequate protection.

SUMMARY OF THE INVENTION

The present invention proposes an electrical supply unit which is arranged to deliver electrical power to one or more electrical outlets and which includes a housing having an incoming electrical supply with live, neutral and earth lines protected by an RCD, in which the unit incorporates a thermal latch which responds when the rate of temperature increase within the housing exceeds a predetermined figure.

For optimum protection the rate-of-rise sensor is preferably provided in addition to a temperature level sensor so that thermal latch may also respond if the temperature within the housing exceeds a predetermined level.

The thermal latch is preferably arranged to activate a warning device. In a preferred configuration the thermal latch does not reset automatically so that the warning device continues to operate to alert people to the fact that an alarm condition has occurred.

The thermal latch is preferably arranged to produce a current flow into the earth or neutral line in order to activate the RCD and thereby electrically isolate the unit from the incoming supply. In this embodiment the thermal latch preferably operates a switching device such as a relay which in turn produces a current flow into the earth or neutral line.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a wiring diagram of an electrical consumer unit in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to Fig. 1, the electrical consumer unit shown by way of example includes a housing 10 which receives respective live, neutral and earth lines L, N and E, from an incoming alternating current electrical supply. The consumer unit supplies multiple electrical circuits via individual circuit breakers MCB. The circuit breakers are each connected to a common live bus 11 which is in turn fed from the incoming live supply line L via a main RCD 12. The incoming neutral supply N feeds a neutral bus 13 via the RCD, while the earth connections are all made to an earth bus 14.

A thermal latch 15 is arranged to operate an audible and/or visual warning device 16, such as a neon indicator light. The latch 15 and the warning device 16 are connected in series between the incoming live and neutral supply lines L and N respectively, prior to the RCD 12. The latch 15 is normally open circuit and incorporates two thermal sensors: 1. A rate-of-change sensor which monitors the rate of temperature change within the housing 10; 2. A temperature sensor which monitors the temperature level within the housing.

In the event that the rate of temperature increase exceeds a predetermined figure and/or the temperature reaches a predetermined level the latch closes and connects the warning device 16 to the live supply line L so that a warning is generated if the operating temperature within the consumer unit rises too quickly.

The thermal latch 15 is also arranged to operate a switching device in the from of a relay 18. (Other forms of switch such as a thyristor could also be used.) The coil of the relay 18 is connected between the latch 15 and the incoming neutral line N, in parallel with the warning device 16. When the latch 15 is triggered as described the relay closes to connect a current limiting resistor Ri from the live bus 11 to the earth or neutral buses, 14 or 13. Since the resistor is connected downstream of the RCD 12 the leakage current flowing through resistor Ri will cause the main RCD to trip, disconnecting the live bus 11 from the incoming live supply and thereby isolating all circuits connected to the consumer unit. Since the thermal latch 15 is connected before the RCD the warning will continue to operate and give a clear audible and/or visual indication of a possible fault condition so that people in the vicinity are alerted to the heat-related fault condition and can readily identify its location and take suitable action. The thermal latch 15 does not reset automatically so that the warning device continues to operate to alert people to the fact that an alarm condition has occurred even after the fault condition causing the rapid temperature increase has ceased to exist.

It is also possible to raise an alarm at a remote location by using a message generating device which is programmed to send a text message via a cellular telephone network to a predetermined number, or numbers, when the thermal latch 15 responds to a fault condition. Alternatively, the message generating device may be a plug-in unit which is inserted into a wall socket on a protected circuit and sends the warning message in response to a power failure. The device can also be programmed to send an appropriate text message to the recipient/s when a fault condition no longer exists, e.g. if a re-connection is made after a short power cut.

It should be noted that the relay 18 is not required to have a latching function since its only purpose is to trigger the RCD to remove power from the protected circuits. Power cannot be restored until the RCD is reset.

Although in the above example the invention is applied to a consumer unit it will be appreciated that the invention could be applied to other electrical units protected by RCBs such as wall sockets and power outlets, extension sockets connected to a mains supply via a cable and plug, junction boxes, light switches or ceiling roses. In each case the RCB could be within the unit or external.

Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

Claims

CLAIMS1. An electrical supply unit which is arranged to deliver electrical power to one or more electrical outlets and which includes a housing having an incoming electrical supply with live, neutral and earth lines protected by an RCD, in which the unit incorporates a thermal latch which responds when the rate of temperature increase within the housing exceeds a predetermined figure.
2. An electrical supply unit according to Claim 1 in which the thermal latch responds when the temperature within the housing exceeds a predetermined level.
3. An electrical supply unit according to Claim 1 or 2 in which the thermal latch is arranged to activate a warning device.
4. An electrical supply unit according to Claim 3 in which the thermal latch does not reset automatically so that the warning device continues to operate to alert people to the fact that an alarm condition has occurred.
5. An electrical supply unit according to any preceding claim in which the thermal latch is arranged to produce a current flow into the earth or neutral line in order to activate the RCD and thereby electrically isolate the unit from the incoming supply.
6. An electrical supply unit according to Claim S in which the thermal latch operates a switching device which in turn produces a current flow into the earth or neutral line.
7. An electrical supply unit according to Claim 6 in which the switching device comprises a relay.
8. An electrical supply unit substantially as described with reference to the drawings.* * * * * * * *